CN116827438A - Optical communication system and first node - Google Patents

Abstract

An optical communication system and a first node are used to solve the four-wave mixing problem of the optical communication system in the existing technology. It can be applied to fronthaul communication systems or other possible single-fiber bidirectional transmission systems. The optical communication system includes a first node, a second node, and an optical fiber connecting the first node and the second node. The optical fiber is used to transmit N first direction wavelength channels sent from the first node to the second node, and the N first direction wavelength channels sent from the second node to the second node. The N second direction wavelength channels sent by the first node, N is an integer greater than 2. The zero dispersion wavelength ZDW of the optical fiber belongs to a ZDW area, at least two first direction wavelength channels and at least one second direction wavelength channel are located in the ZDW area, or at least two second direction wavelength channels and at least one first direction wavelength channel are located in the ZDW area, by mixing the first direction wavelength channel and the second direction wavelength channel distributed in the ZDW area, the four-wave mixing effect of the optical fiber can be weakened or suppressed.

Description

An optical communication system and a first node

Technical Field

The present application relates to the field of optical communication technology, and in particular to an optical communication system and a first node.

Background Art

With the rapid growth of communication services and the number of users, optical communication technology has developed rapidly. Wavelength division multiplexing (WDM) transmission technology is a relatively effective way to achieve the upgrade and expansion of optical communication systems. Optical fiber is usually used to transmit optical signals in optical communication systems, but there is a zero-dispersion wavelength (ZDW) or zero-dispersion frequency (ZDF) in the optical fiber. There is a serious four-wave mixing for the optical signal transmitted around the ZDW (or ZDF), which will cause the distortion of the transmitted optical signal, thereby reducing the performance of the optical communication system. Among them, the zero-dispersion wavelength refers to the wavelength at which the group velocity dispersion (or group delay dispersion per unit length or second-order dispersion) of the optical fiber is 0. Group velocity dispersion is a phenomenon that the group velocity of light in a transparent medium is related to the frequency or wavelength of light. ZDF = C/ZDW, C is the speed of light.

At present, the commonly used method to suppress four-wave mixing in optical fibers is to reduce the transmission power of the light source that transmits the optical signal. Based on this method, some four-wave mixing can be weakened, but the power margin of the optical signal is also suppressed, and for transmission over long distances, this method still has a more serious four-wave mixing effect (or four-wave mixing damage).

In summary, how to weaken or suppress the four-wave mixing effect in optical communication systems is a technical problem that urgently needs to be solved.

Summary of the invention

The present application provides an optical communication system and a first node, which are used to weaken or suppress four-wave mixing of optical fibers.

In a first aspect, the present application provides an optical communication system, the optical communication system comprising a first node, a second node, and an optical fiber for connecting the first node and the second node, the ZDW of the optical fiber belongs to a ZDW region, or it can also be understood that the ZDF of the optical fiber belongs to a ZDF region, wherein ZDF=C/ZDW, C is the speed of light. The optical fiber is used to transmit N first-direction wavelength channels and N second-direction wavelength channels, N is an integer greater than 2, at least two first-direction wavelength channels and at least one second-direction wavelength channel are located in the ZDW region, or at least two second-direction wavelength channels and at least one first-direction wavelength channel are located in the ZDW region, the first-direction wavelength channels and the second-direction wavelength channels in the ZDW region are mixedly distributed, the first-direction wavelength channels are wavelength channels sent by the first node to the second node, and the second-direction wavelength channels are wavelength channels sent by the second node to the first node.

Among them, the first direction wavelength channels and the second direction wavelength channels in the ZDW area are mixedly distributed, which can also be understood as that in the ZDW area, the number of consecutive adjacent first direction wavelength channels is less than or equal to 2, and the number of consecutive adjacent second direction wavelength channels is less than or equal to 2.

Based on the above scheme, in the ZDW area, by mixing and distributing the first-direction wavelength channels and the second-direction wavelength channels, on the one hand, it helps to increase the interval between the wavelength channels (i.e., the first-direction wavelength channels or the second-direction wavelength channels) that transmit in the same direction; on the other hand, it helps to reduce the number of four-wave mixing formed or can eliminate four-wave mixing. Taking the first-direction wavelength channels as an example, for example, based on the distribution method of the prior art, there are 3 groups of first-direction wavelength channels that can form four-wave mixing, but based on the scheme of the present application, at most 2 groups of first-direction wavelength channels can form four-wave mixing; therefore, based on the above scheme, it helps to weaken or suppress the four-wave mixing effect, thereby suppressing the performance degradation of the optical communication system caused by four-wave mixing.

In a possible implementation, the ZDW of different optical fibers may be different, and the ZDW of a standard single-mode optical fiber belongs to a ZDW region ranging from [1300 nanometers to 1324 nanometers].

In a possible implementation, the distribution of the N first-direction wavelength channels and the N second-direction wavelength channels satisfies any one or more of the following: among the N first-direction wavelength channels, there are no four symmetrical first-direction wavelength channels whose centers are located in the ZDW region; further, among the N second-direction wavelength channels, there are no four symmetrical second-direction wavelength channels whose centers are located in the ZDW region.

By designing that among the N first-direction wavelength channels, there are no four symmetrical first-direction wavelength channels with their centers located in the ZDW region, it is helpful to weaken or suppress the first-direction wavelength channels from producing non-degenerate four-wave mixing (see the explanation of the following terms); by designing that among the N second-direction wavelength channels, there are no four symmetrical second-direction wavelength channels with their centers located in the ZDW region, it is possible to weaken or suppress the second-direction wavelength channels from producing non-degenerate four-wave mixing.

In a possible implementation, the distribution of the N first-direction wavelength channels and the N second-direction wavelength channels satisfies any one or more of the following: among the N first-direction wavelength channels, there are not three first-direction wavelength channels with the same intervals and at least two of which are located in the ZDW area; further, among the N second-direction wavelength channels, there are not three second-direction wavelength channels with the same intervals and at least two of which are located in the ZDW area.

By designing that among the N first-direction wavelength channels, there are not three first-direction wavelength channels with the same intervals and at least two of which are located in the ZDW region, it is helpful to weaken or suppress the first-direction wavelength channels from producing degenerate four-wave mixing (see the explanation of the following terms); by designing that among the N second-direction wavelength channels, there are not three second-direction wavelength channels with the same intervals and at least two of which are located in the ZDW region, it is possible to weaken or suppress the second-direction wavelength channels from producing non-degenerate four-wave mixing.

In order to ensure the normal transmission of wavelength channels in optical fibers, it is necessary to control the intervals between wavelength channels. If the wavelength interval is too small, it is easy to cause mutual interference. If the wavelength interval is too large, the utilization rate of the optical fiber will be reduced. In a possible implementation, the minimum frequency interval of the mixed N first-direction wavelength channels and the N second-direction wavelength channels is equal to 800 gigahertz (GHz). In other words, the minimum frequency interval of 2N wavelength channels (including N first-direction wavelength channels and N second-direction wavelength channels) is equal to about 800 GHz. It should be noted that the interval between wavelength channels can be characterized by frequency interval, or can also be characterized by wavelength interval. Here, the frequency interval is used as an example.

In another possible implementation manner, the minimum frequency interval between the mixedly distributed N first-direction wavelength channels and the N second-direction wavelength channels is equal to 400 GHz.

By setting the minimum frequency interval to 400 GHz, the normal transmission of the wavelength channel in the optical fiber can be guaranteed, and the wavelength band can be fully utilized, which helps to improve the transmission capacity of the optical fiber and improve the utilization efficiency of optical fiber resources.

The following exemplarily shows four distribution schemes of N first-direction wavelength channels and N second-direction wavelength channels. These four distribution schemes can all achieve the goal of weakening or suppressing the four-wave mixing effect of the optical fiber.

Solution 1: N first-direction wavelength channels and N second-direction wavelength channels are alternately distributed (or referred to as cross distribution).

In a possible implementation, the intervals of the N first-direction wavelength channels are the same, and the wavelength intervals of the N second-direction wavelength channels are the same. Further, the intervals of the first-direction wavelength channels are the same as the wavelength intervals of the second-direction wavelength channels.

Taking N=6 as an example, the N first direction wavelength channels and the N second direction wavelength channels are distributed in the order of wavelength from small to large as follows: first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel. It can also be understood that the order of the 12 wavelength channels in the order of wavelength from small to large is: the first is the first direction wavelength channel, the second is the second direction wavelength channel, the third is the first direction wavelength channel, the fourth is the second direction wavelength channel, the fifth is the first direction wavelength channel, the sixth is the second direction wavelength channel, the seventh is the first direction wavelength channel, the eighth is the second direction wavelength channel, the ninth is the first direction wavelength channel, the tenth is the second direction wavelength channel, the eleventh is the first direction wavelength channel, and the twelfth is the second direction wavelength channel.

Alternatively, the N first direction wavelength channels and the N second direction wavelength channels are distributed in the order of wavelength from small to large as follows: second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel. In other words, the 12 wavelength channels are distributed in the order of wavelength from small to large as follows: the first is the second direction wavelength channel, the second is the first direction wavelength channel, the third is the second direction wavelength channel, the fourth is the first direction wavelength channel, the fifth is the second direction wavelength channel, the sixth is the first direction wavelength channel, the seventh is the second direction wavelength channel, the eighth is the first direction wavelength channel, the ninth is the second direction wavelength channel, the tenth is the first direction wavelength channel, the eleventh is the second direction wavelength channel, and the twelfth is the first direction wavelength channel.

Based on the above solution 1, the intervals between the N first-direction wavelength channels for co-directional transmission are increased, thereby helping to reduce the four-wave mixing effect caused by the N first-direction wavelength channels. Similarly, the intervals between the N second-direction wavelength channels for co-directional transmission are also increased, which helps to reduce the four-wave mixing caused by the N second-direction wavelength channels.

In a possible implementation, the wavelength intervals of the N first direction wavelength channels are different, and/or the wavelength intervals of the N second direction wavelength channels are different. Based on this, three possible solutions are exemplarily shown below, namely, Solution 2, Solution 3 and Solution 4.

By designing the N first-direction wavelength channels to have different wavelength intervals and/or the N second-direction wavelength channels to have different wavelength intervals, it is helpful to further weaken the four-wave mixing effect of the optical fiber.

Solution 2: swap the order of a pair of wavelength channels in the above-mentioned solution 1.

Taking N=6 as an example, the N first direction wavelength channels and the N second direction wavelength channels are distributed in the order of wavelength from small to large as follows: first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel. It can also be understood that the first is the first direction wavelength channel, the second is the second direction wavelength channel, the third is the first direction wavelength channel, the fourth is the second direction wavelength channel, the fifth is the first direction wavelength channel, the sixth is the second direction wavelength channel, the seventh is the second direction wavelength channel, the eighth is the first direction wavelength channel, the ninth is the first direction wavelength channel, the tenth is the second direction wavelength channel, the eleventh is the first direction wavelength channel, and the twelfth is the second direction wavelength channel.

Alternatively, taking N=6 as an example, the N first direction wavelength channels and the N second direction wavelength channels are distributed in the order of wavelength from small to large as follows: second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel. Alternatively, it can also be understood that the first is the second direction wavelength channel, the second is the first direction wavelength channel, the third is the second direction wavelength channel, the fourth is the first direction wavelength channel, the fifth is the second direction wavelength channel, the sixth is the first direction wavelength channel, the seventh is the first direction wavelength channel, the eighth is the second direction wavelength channel, the ninth is the second direction wavelength channel, the tenth is the first direction wavelength channel, the eleventh is the second direction wavelength channel, and the twelfth is the first direction wavelength channel.

Solution 3, swapping the order of the two pairs of wavelength channels in the above solution 1.

Taking N=6 as an example, the N first direction wavelength channels and the N second direction wavelength channels are distributed in the order of wavelength from small to large as follows: first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel. It can also be understood that the first is the first direction wavelength channel, the second is the second direction wavelength channel, the third is the second direction wavelength channel, the fourth is the first direction wavelength channel, the fifth is the first direction wavelength channel, the sixth is the second direction wavelength channel, the seventh is the first direction wavelength channel, the eighth is the second direction wavelength channel, the ninth is the second direction wavelength channel, the tenth is the first direction wavelength channel, the eleventh is the first direction wavelength channel, and the twelfth is the second direction wavelength channel.

Alternatively, the N first direction wavelength channels and the N second direction wavelength channels are distributed in the order of wavelength from small to large as follows: second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel. It can also be understood that the first is the second direction wavelength channel, the second is the first direction wavelength channel, the third is the first direction wavelength channel, the fourth is the second direction wavelength channel, the fifth is the second direction wavelength channel, the sixth is the first direction wavelength channel, the seventh is the second direction wavelength channel, the eighth is the first direction wavelength channel, the ninth is the first direction wavelength channel, the tenth is the second direction wavelength channel, the eleventh is the second direction wavelength channel, and the twelfth is the first direction wavelength channel.

Solution 4, swapping the order of the three pairs of wavelength channels in the above-mentioned solution 1.

Taking N=6 as an example, the N first direction wavelength channels and the N second direction wavelength channels are distributed in the order of wavelength from small to large as follows: first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel. It can also be understood that the first is the first direction wavelength channel, the second is the second direction wavelength channel, the third is the second direction wavelength channel, the fourth is the first direction wavelength channel, the fifth is the second direction wavelength channel, the sixth is the first direction wavelength channel, the seventh is the first direction wavelength channel, the eighth is the second direction wavelength channel, the ninth is the second direction wavelength channel, the tenth is the first direction wavelength channel, the eleventh is the second direction wavelength channel, and the twelfth is the first direction wavelength channel.

Alternatively, the N first direction wavelength channels and the N second direction wavelength channels are distributed in the order of wavelength from small to large as follows: second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel. It can also be understood that the first is the second direction wavelength channel, the second is the first direction wavelength channel, the third is the first direction wavelength channel, the fourth is the second direction wavelength channel, the fifth is the first direction wavelength channel, the sixth is the second direction wavelength channel, the seventh is the second direction wavelength channel, the eighth is the first direction wavelength channel, the ninth is the first direction wavelength channel, the tenth is the second direction wavelength channel, the eleventh is the first direction wavelength channel, and the twelfth is the second direction wavelength channel.

If the five wavelength channels with the longest wavelengths among the 12 wavelength channels are located in the ZDW region, the distribution of the six first-direction wavelength channels in the above schemes 2, 3 and 4 all meet the following requirements: among the six first-direction wavelength channels, there are no four symmetrical first-direction wavelength channels whose centers are located in the ZDW region, and among the six first-direction wavelength channels, there are no three first-direction wavelength channels with the same intervals and at least two of them are located in the ZDW region. The distribution of the six second-direction wavelength channels all meets the following requirements: among the six second-direction wavelength channels, there are no four symmetrical second-direction wavelength channels whose centers are located in the ZDW region, and among the six second-direction wavelength channels, there are no three second-direction wavelength channels with the same intervals and at least two of them are located in the ZDW region. Therefore, based on the above schemes 2, 3 and 4, they are all helpful to weaken or suppress the four-wave mixing of the optical fiber.

In a possible implementation, the first node includes N first optical transceivers and a first combiner/demultiplexer, and the second node includes N second optical transceivers and a second combiner/demultiplexer; a first end of the first combiner/demultiplexer is connected to the N first optical transceivers, and a second end of the first combiner/demultiplexer is connected to an optical fiber between the first node and the second node; a first end of the second combiner/demultiplexer is connected to the N second optical transceivers, and a second end of the second combiner/demultiplexer is connected to an optical fiber between the first node and the second node.

In a possible implementation, the number of ports at the first end and the second end of the first combiner/demultiplexer is N:1, and the number of ports at the first end and the second end of the second combiner/demultiplexer is N:1.

Transmitting 2N wavelength channels (including N first direction wavelength channels and N second direction wavelength channels) through N ports of a combiner/demultiplexer (including the first combiner/demultiplexer and the second combiner/demultiplexer) helps save 50% of the ports of the combiner/demultiplexer.

In a possible implementation, a first optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel, which are adjacent to the first direction wavelength channel and the second direction wavelength channel corresponding to the same first optical transceiver; and/or, a second optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel, which are adjacent to the first direction wavelength channel and the second direction wavelength channel corresponding to the same second optical transceiver. In other words, N first optical transceivers and N second optical transceivers can realize the transmission of 2N wavelength channels (including N first direction wavelength channels and N second direction wavelength channels).

By making adjacent first direction wavelength channels and second direction wavelength channels correspond to the same first optical transceiver, and/or making adjacent first direction wavelength channels and second direction wavelength channels correspond to the same second optical transceiver, it helps to reduce the number of first optical transceivers and/or the number of second optical transceivers included in the optical communication system.

In a possible implementation manner, the first optical transceiver includes a first optical fiber circulator, and the second optical transceiver includes a second optical fiber circulator.

Since the structure of the fiber optic circulator is relatively simple, using the fiber optic circulator as an optical transceiver helps to reduce the size of the optical communication system.

In a possible implementation manner, the first node includes a distribution unit (DU) and/or a central unit (CU), and the second node includes an active antenna unit (AAU).

In a second aspect, the present application provides a first node, which includes N first optical transceivers, and the N first optical transceivers are used to send N first direction wavelength channels and to receive N second direction wavelength channels, where N is an integer greater than 2; wherein the N first direction wavelength channels and the N second direction wavelength channels are transmitted through optical fiber, a ZDW of the optical fiber belongs to a ZDW area, at least two first direction wavelength channels or at least two second direction wavelength channels are located in the ZDW area, and the first direction wavelength channels and the second direction wavelength channels in the ZDW area are mixedly distributed, the first direction wavelength channel is a wavelength channel sent by the first node to the second node, and the second direction wavelength channel is a wavelength channel sent by the second node to the first node.

In a possible implementation, the first node further includes a first combiner/demultiplexer, a first end of the first combiner/demultiplexer is connected to the N first optical transceivers, and a second end of the first combiner/demultiplexer is used to connect to the optical fiber.

In a possible implementation manner, a first optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel, which are adjacent to the first direction wavelength channel and the second direction wavelength channel corresponding to the same first optical transceiver.

In a possible implementation manner, the first optical transceiver includes a first fiber circulator.

In a possible implementation manner, the number of ports of the first end and the second end of the first combiner/demultiplexer is N:1.

In a third aspect, the present application provides a second node, which includes N second optical transceivers, where the N second optical transceivers are used to receive N first-direction wavelength channels and to send N second-direction wavelength channels, where N is an integer greater than 2; wherein the N first-direction wavelength channels and the N second-direction wavelength channels are transmitted through optical fiber, a ZDW of the optical fiber belongs to a ZDW area, at least two first-direction wavelength channels or at least two second-direction wavelength channels are located in the ZDW area, and the first-direction wavelength channels and the second-direction wavelength channels in the ZDW area are mixedly distributed, the first-direction wavelength channel is a wavelength channel sent by the first node to the second node, and the second-direction wavelength channel is a wavelength channel sent by the second node to the first node.

In a possible implementation, the second node further includes a second combiner/demultiplexer, a first end of the second combiner/demultiplexer is connected to the N second optical transceivers, and a second end of the second combiner/demultiplexer is used to connect to the optical fiber.

In a possible implementation manner, a second optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel, and is adjacent to the first direction wavelength channel and the second direction wavelength channel corresponding to the same second optical transceiver.

In a possible implementation manner, the second optical transceiver includes a second optical fiber circulator.

In a possible implementation manner, the number of ports of the first end and the second end of the second combiner/demultiplexer is N:1.

The technical effects that can be achieved in any of the second to third aspects mentioned above can refer to the description of the beneficial effects in the first aspect mentioned above, and will not be repeated here.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1a is a schematic diagram of the principle of a non-degenerate four-wave mixing provided by the present application;

FIG1b is a schematic diagram of the principle of a degenerate four-wave mixing provided by the present application;

FIG2a is a schematic diagram of a possible application scenario provided by the present application;

FIG2b is a schematic diagram of another possible application scenario provided by the present application;

FIG3 is a schematic diagram of the distribution of wavelength channels provided by the present application;

FIG4 is a schematic diagram of another principle of generating four-wave mixing provided by the present application;

FIG5 is a schematic diagram of the architecture of an optical communication system provided by the present application;

FIG6a is a schematic diagram of the distribution of wavelength channels provided by the present application;

FIG6b is a schematic diagram of distribution of another wavelength channel provided by the present application;

FIG7a is a schematic diagram of distribution of another wavelength channel provided by the present application;

FIG7b is a schematic diagram of distribution of another wavelength channel provided by the present application;

FIG8a is a schematic diagram of distribution of another wavelength channel provided by the present application;

FIG8b is a schematic diagram of distribution of another wavelength channel provided by the present application;

FIG9a is a schematic diagram of distribution of another wavelength channel provided by the present application;

FIG9b is a schematic diagram of distribution of another wavelength channel provided by the present application;

FIG10 is a schematic diagram of a connection relationship between a first node and a second node provided by the present application;

FIG11 is a schematic diagram of the structure of a fiber circulator provided by the present application;

FIG. 12 is a schematic diagram of the connection relationship between a first fiber optic circulator and a second fiber optic circulator provided in the present application.

DETAILED DESCRIPTION

The embodiments of the present application will be described in detail below with reference to the accompanying drawings.

Below, some terms in this application are explained. It should be noted that these explanations are for the convenience of understanding by those skilled in the art and do not constitute a limitation on the scope of protection claimed in this application.

1. Four-wave mixing (FWM)

Four-wave mixing (FWM) is a nonlinear effect based on third-order optical nonlinearity (described by the X ⁽³⁾ coefficient). Four-wave mixing may occur when at least two lights of different frequencies (or wavelengths) propagate in a nonlinear medium (such as an optical fiber). Please refer to Figure 1a. Taking two lights with frequencies _v1 and _v2 ( _v2 > _v1 ) as an example, due to the refractive index modulation of the difference frequency, two new frequencies will be generated, namely: _v3 = _v1- ( _v2 - _v1 ) = _2v1 - _v2 and _v4 = _v2 + ( _v2 - _v1 ) = _2v2 - _v1 . Therefore, if there is light with a frequency of _v3 or _v4 , it will appear that the frequencies of _v3 and _v4 are amplified, that is, the two frequencies of light undergo parametric amplification, thereby affecting and interfering with the original light with a frequency of _v3 or _v4 .

When four-wave mixing involves four different frequencies (or wavelengths), it is called non-degenerate four-wave mixing (or non-degenerate four-wave mixing), see Figure 1a. When two of the four frequencies involved in four-wave mixing overlap, it is called degenerate four-wave mixing, see Figure 1b.

The four-wave mixing effect is usually harmful in optical fiber communications, especially in wavelength division multiplexing technology, where the four-wave mixing effect may cause crosstalk between channels of different wavelengths and/or imbalance of channel power.

2. Single-mode optical fiber

Single-mode fiber refers to an optical fiber that supports only one propagation mode (i.e., self-consistent electric field distribution in the optical fiber) in each polarization direction under a given wavelength. It usually has a relatively small core (diameter of, for example, several microns), and a small refractive index difference between the core and the cladding. The mode radius (generally characterized by the lateral range of the light intensity distribution) is usually several microns (um).

3. O-Band

Not all wavelengths of light are suitable for optical fiber communication. Different wavelengths of light have different transmission losses in optical fibers. Generally, light in the low-loss wavelength region (1260nm to 1625nm) is suitable for optical fiber communication. Light in the low-loss wavelength region can be further divided into five bands, namely O-band (1260nm to 1360nm), E-band (1360nm to 1460), S-band (1460nm to 1530nm), C-band (1530nm to 1565nm) and L-band (1565nm to 1625nm).

Since standard single-mode optical fiber has low dispersion, minimal signal distortion, and low loss in the O-band, its nonlinearity is difficult to control, and some wavelengths may produce obvious four-wave mixing effects near the ZDW of the optical fiber.

4. Wavelength division multiplexing (WDM)

Wavelength division multiplexing refers to the technology of combining two or more different wavelengths of light (carrying various information) at the transmitting end through a multiplexer (also called a combiner) and coupling them into the same optical fiber for transmission; at the receiving end, the demultiplexer (also called a splitter or demultiplexer) separates the various wavelengths of light, and then the optical receiver further processes them to restore the original signal. This technology of simultaneously transmitting two or more different wavelengths of light in the same optical fiber is called wavelength division multiplexing. Wavelength division multiplexing technology can increase the transmission capacity of optical fiber.

5. Wavelength Channel

An optical carrier of a given wavelength carrying a signal is called a wavelength channel. Optical carriers of different wavelength channels have different wavelengths, and the effective signals carried by the optical carriers of different wavelength channels may be the same or different.

The foregoing text introduces some of the terms involved in this application. The following text introduces possible application scenarios of this application.

As shown in Figure 2a, it is a schematic diagram of a possible application scenario provided by the present application. This application scenario takes a fronthaul communication system as an example. The fronthaul communication system includes a central office device 201, a terminal device (or remote device) 202, and an optical fiber for connecting the central office device 201 and the terminal device 202. Different wavelength channels can be transmitted between the central office device 201 and the terminal device 202 through an optical fiber, and the optical fiber can also be called a trunk optical fiber (or feeder optical fiber or fronthaul optical fiber). The length of the optical fiber is usually greater than 5 kilometers. Exemplarily, the direction in which the wavelength channel is transmitted from the central office device 201 to the terminal device 202 can be called the downlink direction, and the direction in which the wavelength channel is transmitted from the terminal device 202 to the central office device 201 can be called the uplink direction.

The local terminal device 201 includes a baseband unit 2011 and a multiplexer/de-multiplexer (MUX/DMUX) 2012 (or called a wavelength division multiplexer/demultiplexer, or a fronthaul aggregation unit (FAU), or a passive multiplexer/demultiplexer). The baseband unit 2011 refers to a communication device or functional module with a baseband signal processing function. The baseband unit 2011 can be, for example, a CU+DU, or a baseband processing unit (building base band unit, BBU), etc. The MUX/DMUX 2012 transmits wavelength channels based on WDM technology. Specifically: at least two wavelength channels of different wavelengths from the baseband unit 2011 are combined together through a combiner, coupled into a trunk optical fiber, and transmitted to the terminal device 202 through the trunk optical fiber; and/or, at least two wavelength channels of different wavelengths combined from the trunk optical fiber are separated by a demultiplexer to obtain each independent wavelength channel. Furthermore, the local end device 201 may also include active equipment 2013, which includes but is not limited to an optical transport network (OTN). OTN is composed of optical network elements connected by a series of optical fiber links, and can provide functions including but not limited to transmission, multiplexing, routing, and short-distance and long-distance transmission of optical channels.

The terminal device 202 includes one or more radio frequency units (RU) 2021 and MUX/DMUX 2022. The radio frequency unit 2021 refers to a communication device or functional module with intermediate frequency signal, radio frequency signal or intermediate radio frequency signal processing function. The radio frequency unit 2021 can be, for example, an active antenna unit (AAU) or a remote radio frequency unit (RRU). One end of the trunk optical fiber is connected to the MUX/DMUX 2012, and the other end of the trunk optical fiber is connected to the MUX/DMUX 2022. The MUX/DMUX 2022 also transmits wavelength channels based on WDM technology. Specifically: at least two wavelength channels of different wavelengths from the radio frequency unit 2021 are combined together through a combiner, coupled into the trunk optical fiber, and transmitted to the local terminal device 201 through the trunk optical fiber; and/or, the at least two wavelength channels of different wavelengths combined from the trunk optical fiber are separated by a splitter to obtain each independent wavelength channel. It should be noted that the combiner and splitter in MUX/DMUX2012 and MUX/DMUX2022 may be integrated together, or may be two independent entities, and this application does not limit this.

In a possible implementation, the central office device 201 may be arranged in a computer room (or central office, CO), which may also include a server, etc. The server is used for computing and information processing, etc. The terminal device 202 is usually arranged at the site side.

It is understandable that the fronthaul communication system may include multiple terminal devices 202 and multiple MUX/DMUX2012, please refer to FIG. 2b, which takes three terminal devices 202 and three MUX/DMUX2012 as an example. Further, the fronthaul communication system may also include two fiber optic distribution boxes, namely, fiber optic distribution box 203 and fiber optic distribution box 204. The first end of the fiber optic distribution box 203 is connected to one end of the three MUX/DMUX2012 in the central office device 201 through three trunk optical fibers, and the second end of the fiber optic distribution box 203 is connected to the first end of the fiber optic distribution box 204 through two trunk optical fibers, and is connected to one terminal device 202 through one trunk optical fiber. The second end of the fiber optic distribution box 204 is connected to two terminal devices 202 through two trunk optical fibers.

It should be noted that the scenarios given above are intended to more clearly illustrate the technical solutions of the present application and do not constitute a limitation on the technical solutions provided by the present application. The optical communication system provided by the present application can also be applied in a variety of other possible scenarios. For example, it can also be applied to other possible short-distance communication systems other than wireless access communication systems based on centralized processing, collaborative radio and real-time cloud processing (referred to as C-RAN systems), such as ultra-high reliability and low latency (URLLC) communication systems, highly synchronized metropolitan area access network systems, multi-access edge computing (MEC) systems, or interconnection systems within data centers.

In one possible implementation, the optical communication system in the present application can support at least one of 50 Gigabyte/second (Gb/s) 4-level pulse amplitude modulation (PAM-4), non-return-to-zero (NRZ) modulation, or polarization multiplexing coherent modulation, where Gb/s represents the transmission rate.

In a possible implementation, the fronthaul communication system may be, for example, a wavelength division multiplexing (LAN WDM) system based on a local area network (LAN) channel, or other possible wavelength division multiplexing systems, which are not listed here one by one.

Taking the LWDM system as an example, LWDM can use 12 bands in the range of 1269nm to 1332nm in the O band. The center frequency, center wavelength and wavelength range of the 12 bands of the LWDM system can be seen in the following Table 1.

Table 1 12-band distribution of LWDM system defined by ITU-T G.owdm

In a possible implementation, the 12 wavelength channels may be referred to as the first wavelength channel (Channel-1, CH1), the second wavelength channel (Channel-2, CH2), the third wavelength channel (Channel-3, CH3), the fourth wavelength channel (Channel-4, CH4), the fifth wavelength channel (Channel-5, CH5), the sixth wavelength channel (Channel-6, CH6), the seventh wavelength channel (Channel-7, CH7), the eighth wavelength channel (Channel-8, CH8), the ninth wavelength channel (Channel-9, CH9), the tenth wavelength channel (Channel-10, CH10), the eleventh wavelength channel (Channel-11, CH11), and the twelfth wavelength channel (Channel-12, CH12) in order of arrival from the smallest wavelength. In other words, CH1 represents the 1st wavelength channel, CH2 represents the 2nd wavelength channel, CH3 represents the 3rd wavelength channel, CH4 represents the 4th wavelength channel, CH5 represents the 5th wavelength channel, CH6 represents the 6th wavelength channel, CH7 represents the 7th wavelength channel, CH8 represents the 8th wavelength channel, CH9 represents the 9th wavelength channel, CH10 represents the 10th wavelength channel, CH11 represents the 11th wavelength channel, and CH12 represents the 12th wavelength channel.

In combination with the above-mentioned Figure 2a or Figure 2b, 6 wavelength channels among the 12 wavelength channels are wavelength channels sent by the central office device 201 to the terminal device 202, or they can also be understood as 6 wavelength channels received by the terminal device 202 from the central office device 201 (can be called first direction wavelength channels), and the first direction is the downstream direction; the other 6 wavelength channels are wavelength channels sent by the terminal device 202 to the central office device 201, or they can also be understood as wavelength channels received by the central office device 201 from the terminal device 202 (can be called second direction wavelength channels), and the second direction is the upstream direction.

A possible wavelength channel distribution can be seen in Figure 3, where the 6 first direction wavelength channels and the 6 second direction wavelength channels are distributed in order from the smallest wavelength to the smallest wavelength: first direction wavelength channel (CH1), first direction wavelength channel (CH2), first direction wavelength channel (CH3), first direction wavelength channel (CH4), first direction wavelength channel (CH5), first direction wavelength channel (CH6), second direction wavelength channel (CH7), second direction wavelength channel (CH8), second direction wavelength channel (CH9), second direction wavelength channel (CH10), second direction wavelength channel (CH11), and second direction wavelength channel (CH12), that is, the distribution of the first direction wavelength channels can be expressed as CH1-CH2-CH3-CH4-CH5-CH6, represented by downward arrows; the distribution of the second direction wavelength channels can be expressed as CH7-CH8-CH9-CH10-CH11-CH12, represented by upward arrows. Further, taking the example that 6 first-direction wavelength channels and 6 second-direction wavelength channels (a total of 12 wavelength channels) can correspond to 6 AAUs (AAU1-AAU2-AAU3-AAU4-AAU5-AAU6 respectively), that is, CH1 inputs AAU1, CH2 inputs AAU2, CH3 inputs AAU3, CH4 inputs AAU4, CH5 inputs AAU5, and CH6 inputs AAU6. CH7 is output from AAU1, CH8 is output from AAU2, CH9 is output from AAU3, CH10 is output from AAU4, CH11 is output from AAU5, and CH12 is output from AAU6.

Based on the distribution of the six first-direction wavelength channels and the six second-direction wavelength channels in FIG3, a relatively serious four-wave mixing effect will be generated. Specifically, if the ZDW of the optical fiber is between CH9 and CH10 (see FIG3), CH8, CH9, CH10, CH11 and CH12 are located in the ZDW region to which the ZDW of the optical fiber belongs, based on the above principle of generating four-wave mixing, it can be determined that CH8, CH9, CH10 and CH11 transmitted in the same direction will interfere with (or be called as damaging) CH9 and CH10, therefore, CH8, CH9, CH10 and CH11 will generate a serious four-wave mixing effect, and the four-wave mixing is non-degenerate four-wave mixing. Moreover, CH7, CH9, CH10 and CH12 transmitted in the same direction will also damage CH9 and CH10, that is, CH7, CH9, CH10 and CH12 will also generate a relatively serious four-wave mixing effect, and the four-wave mixing is also non-degenerate four-wave mixing. Among them, the intervals of the four wavelength channels CH8, CH9, CH10 and CH11 are the same, and the intervals of the four wavelength channels CH7, CH9, CH10 and CH12 are different. Therefore, the intervals of the four wavelength channels that produce non-degenerate four-wave mixing may be the same or different. In other words, the same or different intervals of the four wavelength channels transmitting in the same direction may produce serious four-wave mixing effects.

Please refer to FIG4, which is another schematic diagram of the principle of generating four-wave mixing provided by the present application. If the ZDW of the optical fiber is located at CH9, or it is understood that the ZDW of the optical fiber coincides with CH9, and CH8, CH9, CH10, CH11 and CH12 are located in the ZDW region to which the ZDW of the optical fiber belongs, based on the above introduction to the principle of generating four-wave mixing, it can be determined that CH8, CH9 and CH10 transmitted in the same direction will produce serious four-wave mixing effect, and the four-wave mixing is degenerate four-wave mixing. Moreover, CH7, CH9 and CH11 transmitted in the same direction will produce degenerate four-wave mixing effect.

It should be noted that four-wave mixing may also occur on four interacting wavelength channels outside the ZDW region. In combination with the above FIG. 3 or FIG. 4, for example, CH1, CH2, CH3 and CH4, or CH2, CH3, CH4 and CH5, or CH3, CH4, CH5 and CH6, or CH1, CH3, CH4 and CH6, etc., will all produce non-degenerate four-wave mixing. Moreover, a wavelength channel may also be affected by multiple groups of four-wave mixing effects. For example, CH3 will be affected by the mixing effects of four groups of CH1-CH2-CH3-CH4, CH2-CH3-CH4-CH5, CH3-CH4-CH5-CH6 and CH1-CH3-CH4-CH6.

In view of the above problems, the present application proposes an optical communication system, which can reduce or suppress the four-wave mixing effect, thereby improving the performance of the optical communication system.

Based on the above content, the optical communication system proposed in the present application is specifically described below in combination with Figures 5, 6a to 6b, 7a to 7b, 8a to 8b, 9a to 9b, and 10 to 12.

As shown in FIG5 , it is a schematic diagram of the architecture of an optical communication system provided by the present application. The optical communication system may include a first node, a second node, and an optical fiber for connecting the first node and the second node. Among them, the first node may be the central office device 201 in FIG2a or FIG2b above, and the second node may be the terminal device 202 in FIG2a or FIG2b above. It should be noted that the first node may also include more or less structures than the central office device 201, and the second node may also include more or less structures than the terminal device 202 above, and the present application does not limit this. Alternatively, the first node may be the terminal device 202 in FIG2a or FIG2b above, and the second node may be the central office device 201 in FIG2a or FIG2b above. It can be understood that the first node may also include more or less structures than the terminal device 202 above, and the second node may also include more or less structures than the central office device 201 above, and the present application does not limit this. Among them, the wavelength channel sent by the first node to the second node may be called the first direction wavelength channel, and the wavelength channel sent by the second node to the first node may be called the second direction wavelength channel. In other words, the N first-direction wavelength channels are wavelength channels for transmission in the same direction, and the N second-direction wavelength channels are wavelength channels for transmission in the same direction.

The optical fiber used to connect the first node and the second node can be called a trunk optical fiber, and the ZDW of the optical fiber belongs to a ZDW area. Exemplarily, the optical fiber can be a standard single-mode fiber (SSMF), and the range of the ZDW area to which the ZDW of the standard single-mode optical fiber belongs is [1300nm, 1324nm]. It can also be understood that the ZDW of different optical fibers may be different, and the ZDW of different standard single-mode optical fibers is a ZDW in the ZDW area to which they belong. The optical fiber is used to transmit N first-direction wavelength channels and N second-direction wavelength channels, where N is an integer greater than 2. Among them, at least two first-direction wavelength channels and at least one second-direction wavelength channel are located in the ZDW area, or at least two second-direction wavelength channels and at least one first-direction wavelength channel are located in the ZDW area. The first-direction wavelength channels and the second-direction wavelength channels in the ZDW area are mixedly distributed, which can also be understood as: in the ZDW area, the number of consecutively adjacent first-direction wavelength channels is less than or equal to 2, and the number of consecutively adjacent second-direction wavelength channels is less than or equal to 2. Further, in some possible embodiments, at least two first-direction wavelength channels and at least two second-direction wavelength channels are located in the ZDW region, and the first-direction wavelength channels and the second-direction wavelength channels located in the ZDW region are mixedly distributed.

Based on the above optical communication system, by mixing and distributing the first direction wavelength channel and the second direction wavelength channel in the ZDW area, on the one hand, it helps to increase the interval between the wavelength channels that transmit in the same direction; on the other hand, it helps to reduce the number of four-wave mixing or eliminate four-wave mixing. Taking the first direction wavelength channel as an example, for example, based on the distribution method of the prior art, there are 3 groups of first direction wavelength channels that can form four-wave mixing, but based on the above optical communication system, at most 2 groups of first direction wavelength channels can form four-wave mixing. Based on this, it helps to weaken or suppress the four-wave mixing effect, thereby suppressing the performance degradation of the optical communication system caused by four-wave mixing. Furthermore, since the transmission of the first direction wavelength channel and the transmission of the second direction wavelength channel use the same optical fiber, the first node and the second node have better synchronization quality.

The following describes each structure shown in FIG. 5 to provide an exemplary specific implementation scheme.

1. Fiber Optic

In a possible implementation, the optical fiber is used to transmit N first-direction wavelength channels and N second-direction wavelength channels. The intensity of the four-wave mixing effect in the optical fiber is related to the ZDW (or ZDF) of the optical fiber, and the distribution of the N first-direction wavelength channels and the N second-direction wavelength channels.

The following is an exemplary description of the conditions that the distribution of N first-direction wavelength channels and N second-direction wavelength channels satisfies in order to achieve the reduction or suppression of the four-wave mixing effect of the optical fiber.

Condition 1: Among the wavelength channels propagating in the same direction (the wavelength channels in the first direction or the wavelength channels in the second direction), there are no four symmetrical wavelength channels whose centers (or referred to as symmetry axes or centers) are located in the ZDW region.

Four symmetrical wavelength channels refer to that the wavelengths (or frequencies) of the four wavelength channels are symmetrical about the wavelength centers (or frequency centers) of the four wavelength channels. In conjunction with the above FIG1a, the frequency centers of the four wavelength channels are the centers of _v1 and _v2 (i.e., _v0 = _v1 + _v2 /2), and _v1 , _v2 , _v3 , and _v4 are four symmetrical wavelength channels symmetrical about the frequency center _v0 . In conjunction with the above FIG3, taking CH8, CH9, CH10, and CH11 as examples, the wavelength centers λ0 of CH8, CH9, CH10, and CH11 coincide with the ZDW, and CH8, CH9, CH10, and CH11 are four symmetrical wavelength channels symmetrical about the wavelength center λ0. In order to weaken or suppress the four-wave mixing effect of the optical fiber, in the wavelength channels transmitting in the same direction, there is no distribution such as CH8, CH9, CH10, and CH11.

Specifically, among the N first direction wavelength channels, there are no four symmetrical first direction wavelength channels whose centers are located in the ZDW region; and/or, among the N second direction wavelength channels, there are no four symmetrical second direction wavelength channels whose centers are located in the ZDW region.

Since severe four-wave mixing will occur in the ZDW region of the optical fiber, if the distribution of the N first-direction wavelength channels and the N second-direction wavelength channels meets the above condition 1, four-wave mixing will not occur in the ZDW region. Therefore, the distribution of the N first-direction wavelength channels and the N second-direction wavelength channels that meets the above condition 1 can weaken or suppress the non-degenerate four-wave mixing effect.

Furthermore, four-wave mixing also includes a degenerate four-wave mixing effect. If the distribution of the N first-direction wavelength channels and the N second-direction wavelength channels meets the following condition 2, the degenerate four-wave mixing effect can be weakened or suppressed.

Condition 2: Among the wavelength channels propagating in the same direction, there are no three wavelength channels with the same interval and at least two of which are located in the ZDW region.

In a possible implementation, the interval between wavelength channels can be characterized by frequency interval or wavelength interval. In conjunction with the above FIG4, taking CH8, CH9 and CH10 as an example, the wavelength intervals of these three wavelength channels are the same, and CH8, CH9 and CH10 are all located in the ZDW region. In order to weaken or suppress the four-wave mixing of the optical fiber, in the wavelength channels of the same direction transmission, there is no distribution such as CH8, CH9 and CH10.

Specifically, among the N first-direction wavelength channels, there are no three first-direction wavelength channels with the same intervals and at least two of which are located in the ZDW region; and/or, among the N second-direction wavelength channels, there are no three second-direction wavelength channels with the same intervals and at least two of which are located in the ZDW region. It can also be understood that among the N first-direction wavelength channels, there are no: three first-direction wavelength channels with the same intervals and two of these three first-direction wavelength channels with the same intervals are located in the ZDW region; and/or, among the N second-direction wavelength channels, there are no: three second-direction wavelength channels with the same intervals and two of these three second-direction wavelength channels with the same intervals are located in the ZDW region.

It can be understood that if the distribution of the N first-direction wavelength channels only satisfies the above condition 1, the non-degenerate four-wave mixing effect in the first direction can be weakened or suppressed; if the distribution of the N first-direction wavelength channels only satisfies the above condition 2, the degenerate four-wave mixing effect in the first direction can be weakened or suppressed; if the distribution of the N first-direction wavelength channels satisfies both the above condition 1 and the above condition 2, both the non-degenerate four-wave mixing effect in the first direction and the degenerate four-wave mixing effect in the first direction can be weakened or suppressed. Similarly, the distribution of the N second-direction wavelength channels can also weaken or suppress the non-degenerate four-wave mixing effect and/or the degenerate four-wave mixing effect, which will not be described in detail here.

In a possible implementation, the four-wave mixing effect in the optical fiber may be weakened by increasing the interval between adjacent wavelength channels, especially increasing the interval between adjacent wavelength channels in the ZDW region.

Through simulation, it can be determined that due to the influence of polarization-mode dispersion (PMD), the polarization states of the wavelength channels that interact with each other will change differently during the transmission of the wavelength channels in the optical fiber. Since the four-wave mixing effect is strongest when the polarization states of the wavelength channels that produce four-wave mixing are the same, the four-wave mixing effect will be weakened if the polarization states of the interacting wavelength channels are different. For example, the polarization mode dispersion coefficient of the optical fiber is 0.1 picosecond/square root kilometer. The average differential group delay (DGD) caused by polarization mode dispersion is about 0.22ps when the wavelength channel is transmitted through 5km of optical fiber. This means that during the 5km transmission process, the relative polarization of two wavelength channels separated by about 4.5THz changes by an average of 360 degrees. It can also be understood that when the interval between two wavelength channels transmitted in the same direction is 4.5THz, the polarization state changes by an average of 360 degrees, which can weaken or suppress the four-wave mixing effect.

Based on this, when the interval between the two wavelength channels with the largest interval in the co-propagating wavelength channels is greater than or equal to 4.5 THz, the above conditions 1 and 2 can be appropriately relaxed. For example, condition 1 can be relaxed to the following condition 3, and condition 2 can be relaxed to the following condition 4. In conjunction with the above FIG3, CH8, CH9, CH10, CH11 and CH12 are located in the ZDW region to which the ZDW of the optical fiber belongs. When the interval between CH8 and CH12 is greater than or equal to 4.5 THz, the above condition 1 can be relaxed to the following condition 3, and the above condition 2 can be relaxed to the following condition 4, which can also achieve the weakening or suppression of four-wave mixing in the optical fiber.

Condition 3: the interval between the two wavelength channels with the largest interval among the wavelength channels propagating in the same direction in the ZDW region is greater than or equal to a threshold, and a maximum of four symmetrical wavelength channels with their centers located in the ZDW region are allowed among the wavelength channels propagating in the same direction.

The threshold value may be equal to or greater than 4.5 THz, for example.

Specifically, among the N first direction wavelength channels, at most four symmetrical first direction wavelength channels are allowed to have centers located in the ZDW region; and/or, among the N second direction wavelength channels, at most four second direction wavelength channels are allowed to have centers located symmetrically in the ZDW region.

Condition 4: the interval between the two wavelength channels with the largest interval among the wavelength channels propagating in the same direction in the ZDW region is greater than or equal to the threshold. Among the wavelength channels propagating in the same direction, at most three wavelength channels with the same interval and at least two of them are located in the ZDW region are allowed to exist.

Specifically, among the N first-direction wavelength channels, at most three first-direction wavelength channels with the same intervals and at least two of which are located in the ZDW area are allowed to exist; and/or, among the N second-direction wavelength channels, at most three second-direction wavelength channels with the same intervals and at least two of which are located in the ZDW area are allowed to exist. It can also be understood that among the N first-direction wavelength channels, at most three first-direction wavelength channels are allowed to satisfy: the same intervals and at least two of the three first-direction wavelength channels with the same intervals are located in the ZDW area; and/or, among the N second-direction wavelength channels, at most three second-direction wavelength channels are allowed to satisfy: the same intervals and at least two of the three second-direction wavelength channels with the same intervals are located in the ZDW area.

In order to ensure that the optical fiber transmits wavelength channels normally, it is necessary to control the interval between each wavelength channel. If the interval is too small, it is easy to cause mutual interference between the wavelength channels. If the interval is too large, the utilization rate of the optical fiber will be reduced. In a possible implementation, the minimum frequency interval of the mixed N first-direction wavelength channels and the N second-direction wavelength channels is equal to 800 GHz. It can also be understood that the minimum frequency interval between two adjacent wavelength channels in the 2N wavelength channels (including N first-direction wavelength channels and N second-direction wavelength channels) is equal to 800 GHz. In another possible implementation, the minimum frequency interval of the mixed N first-direction wavelength channels and the N second-direction wavelength channels is equal to 400 GHz. It can also be understood that the minimum frequency interval between two adjacent wavelength channels in the 2N wavelength channels (including N first-direction wavelength channels and N second-direction wavelength channels) is equal to 400 GHz. Among them, the two adjacent wavelength channels can be the first direction wavelength channel and the first direction wavelength channel, or can also be the second direction wavelength channel and the second direction wavelength channel, or can also be the first direction wavelength channel and the second direction wavelength channel. By setting the minimum frequency interval to 400 GHz, it helps to make full use of band resources and help increase the transmission capacity of optical fiber, thereby improving the utilization efficiency of optical fiber resources.

In a possible implementation, N can be 6, that is, the optical communication system can use 12 wavelength channels for transmission, and the interval between adjacent wavelength channels after mixing can be 400 GHz or 800 GHz. Alternatively, N can also be equal to 12, that is, the optical communication system can use 24 wavelength channels for transmission, and the interval between adjacent wavelength channels after mixing can also be 400 GHz or 800 GHz, so that the aggregation capacity of the optical fiber can be further increased. It should be noted that N can also have other values, and this application does not limit the specific value of N.

In order to facilitate further understanding of the scheme, N=6, i.e., 6 first-direction wavelength channels and 6 second-direction wavelength channels are used as an example. There are 12 wavelength channels in total, and there are 2^6=64 possible wavelength channel distribution schemes for these 12 wavelength channels, please refer to Table 2. A in Table 2 represents an adjacent pair of wavelength channels (the adjacent first-direction wavelength channels and the second-direction wavelength channels corresponding to the same first optical transceiver can be called a pair of wavelength channels, and the adjacent first-direction wavelength channels and the second-direction wavelength channels corresponding to the same second optical transceiver can also be called a pair of wavelength channels, and the details can be referred to in the following related descriptions), in which the wavelength of the first-direction wavelength channel is shorter than the wavelength of the second-direction wavelength channel. B represents an adjacent pair of wavelength channels, in which the wavelength of the first-direction wavelength channel is longer than the wavelength of the second-direction wavelength channel. In other words, the wavelengths are distributed in the order from small to large, and the wavelength channels represented by A are distributed as: first-direction wavelength channels, second-direction wavelength channels. The wavelength channels represented by B are distributed as: second-direction wavelength channels, first-direction wavelength channels. 1 represents the first-direction wavelength channel, and 0 represents the second-direction wavelength channel.

Table 2 Distribution of 6 first direction wavelength channels and 6 second direction wavelength channels

It should be noted that any of the 64 mixed distributions of 12 wavelength channels given in Table 2 above can reduce or suppress four-wave mixing compared to the distributions in Figure 3 or Figure 4. In order to further weaken or suppress the four-wave mixing of the optical fiber, the N first-direction wavelength channels and the N second-direction wavelength channels also need to meet the above conditions 3 and/or 4. In order to further weaken or suppress the four-wave mixing of the optical fiber, the N first-direction wavelength channels and the N second-direction wavelength channels also need to meet the above conditions 1 and/or 2.

In combination with the above Table 2, taking N=6 as an example, four schemes for distributing 6 first-direction wavelength channels and 6 second-direction wavelength channels are given as examples. The distribution of the 6 first-direction wavelength channels and the 6 second-direction wavelength channels in these four schemes satisfies the above Conditions 1 and 2, and can effectively weaken or suppress the four-wave mixing effect.

In the following description, the first direction wavelength channel and the second direction wavelength channel in each rectangular frame in FIG. 6a , FIG. 6b , FIG. 7a , FIG. 7b , FIG. 8a , FIG. 8b , FIG. 9a and FIG. 9b may be referred to as a wavelength channel pair.

Solution 1: 6 first direction wavelength channels and 6 second direction wavelength channels are alternately distributed (or called cross distribution).

It can also be understood that the wavelength channel in the second direction is adjacent to the wavelength channel in the first direction, and the wavelength channel in the first direction is adjacent to the wavelength channel in the second direction.

In a possible implementation, the interval _Δ11 of the six first direction wavelength channels is the same, and the wavelength interval _Δ21 of the six second direction wavelength channels is the same. Further, the wavelength interval _Δ11 of the first direction wavelength channel is the same as the wavelength interval of the second direction wavelength channel, as shown in FIG. 6a below.

Based on whether the wavelength of the wavelength channel in the first direction of the wavelength channel pair is longer or shorter than that of the wavelength channel in the second direction, the following two solutions can be used.

Solution 1.1: In a pair of wavelength channels, the wavelength of the wavelength channel in the first direction is shorter than the wavelength of the wavelength channel in the second direction.

Please refer to FIG. 6a, which is a schematic diagram of the distribution of a wavelength channel provided by the present application. The schematic diagram of the distribution of the wavelength channel corresponds to the sequence number 1 in the above Table 2. Adjacent CH1 and CH2 are a wavelength channel pair, adjacent CH3 and CH4 are a wavelength channel pair, adjacent CH5 and CH6 are a wavelength channel pair, adjacent CH7 and CH8 are a wavelength channel pair, adjacent CH9 and CH10 are a wavelength channel pair, and adjacent CH11 and CH12 are a wavelength channel pair. The 6 first-direction wavelength channels and the 6 second-direction wavelength channels are distributed in the order of wavelength from small to large: first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel. Or it can also be expressed as, the 6 first direction wavelength channels are distributed on the 1st, 3rd, 5th, 7th, 9th and 11th wavelength channels, which can be expressed as: CH1-CH3-CH5-CH7-CH9-CH11; the 6 second direction wavelength channels are distributed on the 2nd, 4th, 6th, 8th, 10th and 12th wavelength channels, which can be expressed as: CH2-CH4-CH6-CH8-CH10-CH12.

Through the above scheme 1.1, there is a set of first wavelength channel distributions (i.e., CH6-CH8-CH10-CH12) that may cause more serious non-degenerate four-wave mixing. However, since the intervals between the N first-direction wavelength channels are doubled compared to FIG. 3, and since increasing the intervals between the wavelength channels can also weaken the four-wave mixing effect, the distribution based on the above scheme 1.1 can weaken the four-wave mixing effect of the N first-direction wavelength channels in the optical fiber. Similarly, the four-wave mixing effect (or four-wave mixing damage) of the N second-direction wavelength channels in the optical fiber can also be weakened.

Solution 1.2: In a pair of wavelength channels, the wavelength of the wavelength channel in the first direction is longer than the wavelength of the wavelength channel in the second direction.

Please refer to Figure 6b, which is a schematic diagram of the distribution of another wavelength channel provided by the present application. The schematic diagram of the distribution of the wavelength channel corresponds to the serial number 33 in the above Table 2. For the introduction of the wavelength channel pair, please refer to the description in the above scheme 1.1, which will not be repeated here. The 6 first-direction wavelength channels and the 6 second-direction wavelength channels are distributed in the order of wavelength from small to large as follows: second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel. It can also be expressed as: the distribution of the 6 first-direction wavelength channels: CH2-CH4-CH6-CH8-CH10-CH12, that is, the 6 first-direction wavelength channels are distributed on the 2nd, 4th, 6th, 8th, 10th and 12th wavelength channels. The distribution mode of the 6 second direction wavelength channels is: CH1-CH3-CH5-CH7-CH9-CH11, that is, the 6 second direction wavelength channels are distributed on the 1st, 3rd, 5th, 7th, 9th and 11th wavelength channels.

Through the above solution 1.2, the four-wave mixing effect of the N first-direction wavelength channels and the N second-direction wavelength channels in the optical fiber can also be weakened. For specific analysis, please refer to the introduction of the above solution 1.1, which will not be repeated here.

In a possible implementation, the interval Δ between adjacent wavelength channels (i.e., the first direction wavelength channel and the second direction wavelength channel) of the mixed distribution is equal to 800 GHz. In conjunction with Figure 6a or Figure 6b above, the interval Δ between CH1 and CH2 is equal to 800 GHz, and the interval Δ between CH2 and CH3 is also equal to 800 GHz. Alternatively, the interval Δ between two adjacent wavelength channels (i.e., the first direction wavelength channel and the second direction wavelength channel) of the mixed distribution is equal to 400 GHz. In conjunction with Figure 6a above, the interval Δ between CH1 and CH2 is equal to 400 GHz, and the interval Δ between CH2 and CH3 is also equal to 400 GHz.

Further, in FIG. 6a or FIG. 6b , the interval _Δ11 between adjacent first direction wavelength channels is equal to twice the interval Δ between adjacent wavelength channels of the mixed distribution, and the interval _Δ21 between adjacent second direction wavelength channels is equal to twice the interval Δ between adjacent wavelength channels of the mixed distribution.

Solution 2: The intervals between the six wavelength channels propagating in the same direction are different. Specifically, the first direction and the second direction of a wavelength channel pair in FIG. 6a or FIG. 6b are interchanged (or flipped).

Specifically, the six first direction wavelength channels have different intervals, and the six second direction wavelength channels have different intervals. Further, among the six first direction wavelength channels, the intervals of the first direction wavelength channels located in the ZDW region are different, and among the six second direction wavelength channels, the intervals of the second direction wavelength channels located in the ZDW region are different.

Based on whether the wavelength of the first direction wavelength channel in the first wavelength channel pair is longer or shorter than that of the second direction wavelength channel, the following two solutions can be divided.

Solution 2.1: The wavelength of the wavelength channel in the first direction is shorter than the wavelength of the wavelength channel in the second direction.

Please refer to Figure 7a, which is a distribution diagram of another wavelength channel provided by the present application. The distribution diagram of the wavelength channel corresponds to the serial number 5 in the above Table 2. For the introduction of the wavelength channel pair, please refer to the description in the above scheme 1.1, which will not be repeated here. The 6 first-direction wavelength channels and the 6 second-direction wavelength channels are distributed in the order of wavelength from small to large as follows: first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel. Or it can also be expressed as, the distribution of the 6 first-direction wavelength channels is: CH1-CH3-CH5-CH8-CH9-CH11, and the distribution of the 6 second-direction wavelength channels is: CH2-CH4-CH6-CH7-CH10-CH12. It can be understood that the above Figure 7a is equivalent to exchanging the first-direction and second-direction order of CH7 and CH8 in the above Figure 6a. Specifically, the 7th wavelength channel in Figure 6a is the first direction wavelength channel, and the 7th wavelength channel in Figure 7a is the second direction wavelength channel. The 8th wavelength channel in Figure 6a is the second direction wavelength channel, and the 8th wavelength channel in Figure 7a is the first direction wavelength channel.

Further, the wavelength intervals of the six first direction wavelength channels are different, and the wavelength intervals of the six second direction wavelength channels are also different. Referring to FIG. 7a, the wavelength intervals of the six first direction wavelength channels have three sizes, namely, wavelength intervals Δ ₁₁ , Δ ₁₂ , and Δ ₁₃ , and the wavelength intervals of the six second direction wavelength channels also include three sizes, namely, Δ ₂₁ , Δ ₂₂ , and Δ ₂₃ . Further, among the six first direction wavelength channels, the intervals of the first direction wavelength channels located in the ZDW region are different, and among the six second direction wavelength channels, the intervals of the second direction wavelength channels located in the ZDW region are different.

In combination with the above Table 1, taking the ZDW region of the standard single-mode optical fiber as an example, since the ZDW region is in CH8, CH9, CH10, CH11 and CH12, the directions of CH7 and CH8 can be flipped to help further weaken four-wave mixing.

Generally, the five wavelength channels with the longest wavelengths among the 12 wavelength channels (CH8, CH9, CH10, CH11 and CH12, respectively) are located in the ZDW region, in order of wavelength distribution from small to large. Therefore, interchanging the first direction and the second direction of the wavelength channels located in the ZDW region helps to further weaken or suppress four-wave mixing. Further, CH8, CH9 and CH11 are located in the ZDW region, and these three first-direction wavelength channels will not produce four-wave mixing, and the six first-direction wavelength channels satisfy: there are no four symmetrical first-direction wavelength channels whose centers are located in the ZDW region, and there are no three first-direction wavelength channels with the same intervals and at least two of which are located in the ZDW region among the six first-direction wavelength channels. Further, the second-direction wavelength channels located in the ZDW region are CH10 and CH12, and these two second-direction wavelength channels will not produce four-wave mixing either, and the six second-direction wavelength channels also satisfy: there are no four symmetrical second-direction wavelength channels whose centers are located in the ZDW region, and there are no three first-direction wavelength channels with the same intervals and at least two of which are located in the ZDW region. In other words, the distribution of the N first direction wavelength channels and the N second direction wavelength channels based on the above solution 2.1 satisfies the above conditions 1 and 2. Therefore, the distribution of the wavelength channels based on the above solution 2.1 helps to further weaken or suppress the four-wave mixing effect.

It should be noted that among the 12 wavelength channels of the above scheme 2.1, there are still four symmetrical first-direction wavelength channels among the six first-direction wavelength channels, namely, CH1, CH3, CH9, and CH11 (where CH9 and CH11 are in the ZDW region), but the centers of these four symmetrical first-direction wavelength channels are located between CH6 and CH7, resulting in a weaker four-wave mixing effect. Similarly, there are still four symmetrical second-direction wavelength channels among the six second-direction wavelength channels, namely, CH2, CH4, CH10, and CH12, and the centers of these four symmetrical second-direction wavelength channels are located between CH6 and CH7, resulting in a weaker four-wave mixing effect.

Solution 2.2: The wavelength of the wavelength channel in the first direction is longer than the wavelength of the wavelength channel in the second direction.

Please refer to FIG. 7b, which is a schematic diagram of the distribution of another wavelength channel provided by the present application. The schematic diagram of the distribution of the wavelength channel corresponds to the serial number 37 in the above Table 2. For the introduction of the wavelength channel pair, please refer to the description in the above scheme 1.1, which will not be repeated here. The 6 first-direction wavelength channels and the 6 second-direction wavelength channels are distributed in the order of wavelength from small to large as follows: second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel. Or it can also be expressed as the distribution of the 6 first-direction wavelength channels: CH2-CH4-CH6-CH7-CH10-CH12, and the distribution of the 6 second-direction wavelength channels: CH1-CH3-CH5-CH8-CH9-CH11. It can be understood that the above FIG. 7b is equivalent to flipping CH7 and CH8 in the above FIG. 6b in the first direction and the second direction. Specifically, the 7th wavelength channel in Figure 6b is the second direction wavelength channel, and the 7th wavelength channel in Figure 7b is the first direction wavelength channel. The 8th wavelength channel in Figure 6b is the first direction wavelength channel, and the 8th wavelength channel in Figure 7b is the second direction wavelength channel.

The distribution of 6 first direction wavelength channels and 6 second direction wavelength channels based on the above scheme 2.2 can also weaken or suppress the four-wave mixing effect of the optical fiber. For specific analysis, please refer to the introduction of scheme 2.1, which will not be repeated here.

It should be noted that, among the 12 wavelength channels of the above scheme 2.2, there are still four symmetrical second-direction wavelength channels among the six second-direction wavelength channels, namely, CH1, CH3, CH9, and CH11, but the centers of these four symmetrical second-direction wavelength channels are located between CH6 and CH7, and the intervals are large, so the four-wave mixing effect caused is weak. Similarly, there are also four symmetrical first-direction wavelength channels among the six first-direction wavelength channels, namely, CH2, CH4, CH10, and CH12, and the centers of these four symmetrical first-direction wavelength channels are located between CH6 and CH7, and the intervals are also large, so the four-wave mixing effect caused is weak.

Through the above scheme 2, the transmission of the six first-direction wavelength channels and the six second-direction wavelength channels in the optical fiber will not cause serious non-degenerate four-wave mixing (i.e., non-degenerate four-wave mixing). Theoretical simulation found that based on the distribution of the above scheme 2, the performance degradation caused by four-wave mixing can be reduced by about 20 times compared with the distribution (configuration) of Figure 3. Furthermore, under the 20-fold suppression of four-wave mixing, the 12-channel 25Gb/s LWDM system based on NRZ modulation can achieve less than 0.5 decibel (dB) of four-wave mixing after being transmitted over 10km of optical fiber; the 12-channel 50Gb/s LWDM system based on PAM-4 modulation has a performance degradation of less than 2dB after being transmitted over 10km of optical fiber, thereby increasing the aggregate capacity of the optical fiber from 300 Gb/s to 600Gb/s. The LWDM system can also use polarization multiplexing coherent modulation to achieve a single-wavelength channel rate of 100Gb/s or higher.

Solution 3: The intervals of the six wavelength channels propagating in the same direction are different. Specifically, the first direction and the second direction of the two wavelength channel pairs in FIG. 6a or FIG. 6b are flipped (or interchanged).

Based on this, the wavelength intervals of the six first direction wavelength channels are different, and the wavelength intervals of the six second direction wavelength channels are different. Further, among the six first direction wavelength channels, the intervals of the first direction wavelength channels located in the ZDW region are different, and among the six second direction wavelength channels, the intervals of the second direction wavelength channels located in the ZDW region are different.

Based on whether the wavelength of the first direction wavelength channel in the first wavelength channel pair is longer or shorter than that of the second direction wavelength channel, the following two solutions can be divided.

Solution 3.1: The wavelength of the wavelength channel in the first direction is shorter than the wavelength of the wavelength channel in the second direction.

Please refer to FIG8a, which is a schematic diagram of the distribution of another wavelength channel provided by the present application. The schematic diagram of the distribution of the wavelength channel corresponds to the serial number 19 in the above Table 2. The 6 first-direction wavelength channels and the 6 second-direction wavelength channels are distributed in the order of wavelength from small to large as follows: first-direction wavelength channel, second-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel. It can also be expressed as the distribution of the 6 first-direction wavelength channels: CH1-CH4-CH5-CH7-CH10-CH11, and the distribution of the 6 second-direction wavelength channels: CH2-CH3-CH6-CH8-CH9-CH12. For the introduction of the wavelength channel pair, please refer to the description in the above-mentioned scheme 1.1, which will not be repeated here. It can be understood that the above FIG8a is equivalent to swapping the first and second directions of CH3 and CH4 in the above FIG6a, and swapping the first and second directions of CH9 and CH10.

Taking CH8, CH9, CH10, CH11 and CH12 located in the ZDW region as an example, in the ZDW region, the second direction wavelength channels are distributed in the 8th, 9th and 12th, and these three second direction wavelength channels will not produce four-wave mixing; the first direction wavelength channels are distributed in the 10th and 11th, and these two first direction wavelength channels will not produce four-wave mixing. Moreover, the 6 first direction wavelength channels satisfy: there are no four symmetrical first direction wavelength channels whose centers are located in the ZDW region, nor are there: three first direction wavelength channels with the same intervals and at least two of which are located in the ZDW region. The 6 second direction wavelength channels satisfy: there are no four symmetrical second direction wavelength channels whose centers are located in the ZDW region, nor are there: three second direction wavelength channels with the same intervals and at least two of which are located in the ZDW region. It can also be understood that the distribution of the 6 first direction wavelength channels and the 6 second direction wavelength channels based on the above scheme 3.1 satisfies the above conditions 1 and 2. Therefore, the distribution based on the above scheme 3.1 helps to further weaken or suppress the four-wave mixing effect.

It should be noted that, among the 12 wavelength channels of the above scheme 3.1, there are four symmetrical first-direction wavelength channels among the six first-direction wavelength channels, namely, CH1, CH4, CH7, and CH10, and the centers of these four symmetrical first-direction wavelength channels are located between CH5 and CH6, so the four-wave mixing effect caused is relatively weak. Further, there are four symmetrical first-direction wavelength channels among the six first-direction wavelength channels, namely, CH4, CH5, CH10, and CH11, and the centers of these four symmetrical first-direction wavelength channels are located between CH7 and CH8, so the four-wave mixing effect caused is relatively weak. Similarly, there are four symmetrical second-direction wavelength channels among the N second-direction wavelength channels, namely, CH2, CH3, CH8, and CH9, and the centers of these four symmetrical second-direction wavelength channels are located between CH5 and CH6, so the four-wave mixing effect caused is relatively weak. Furthermore, there are four symmetrical second direction wavelength channels among the N second direction wavelength channels, namely CH3, CH6, CH9, and CH12. The centers of these four symmetrical second direction wavelength channels are located between CH7 and CH8. Therefore, the four-wave mixing effect caused is weak.

Solution 3.2: The wavelength of the wavelength channel in the first direction is shorter than the wavelength of the wavelength channel in the second direction.

Please refer to FIG8b, which is a schematic diagram of another wavelength channel distribution provided by the present application. The schematic diagram of the wavelength channel distribution corresponds to the serial number 51 in the above Table 2. The 6 first direction wavelength channels and the 6 second direction wavelength channels are distributed in the order of wavelength from small to large as follows: second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel. Or it can also be expressed as the distribution of the 6 first direction wavelength channels: CH2-CH3-CH6-CH8-CH9-CH12, and the distribution of the 6 second direction wavelength channels: CH1-CH4-CH5-CH7-CH10-CH11. It can be understood that the above FIG8b is equivalent to flipping the first direction and the second direction of CH3 and CH4 in the above FIG6b, and flipping the first direction and the second direction of CH9 and CH10.

The distribution of 6 first direction wavelength channels and 6 second direction wavelength channels based on the above scheme 3.2 can also weaken or suppress the four-wave mixing effect of the optical fiber. For specific analysis, please refer to the introduction of scheme 3.1, which will not be repeated here.

It should be noted that, among the 12 wavelength channels of the above scheme 3.2, there are four symmetrical second direction wavelength channels among the 6 second direction wavelength channels, namely, CH1, CH4, CH7, and CH10. The centers of these four symmetrical second direction wavelength channels are located between CH5 and CH6, and the intervals are large, so the four-wave mixing effect caused is weak. Further, there are four symmetrical second direction wavelength channels among the 6 second direction wavelength channels, namely, CH4, CH5, CH10, and CH11. The centers of these four symmetrical second direction wavelength channels are located between CH7 and CH8, and the intervals are large, so the four-wave mixing effect caused is weak. Similarly, there are four symmetrical first direction wavelength channels among the N first direction wavelength channels, namely, CH2, CH3, CH8, and CH9. The centers of these four symmetrical first direction wavelength channels are located between CH5 and CH6, and the intervals are large, so the four-wave mixing effect caused is weak. Furthermore, there are four symmetrical first direction wavelength channels among the N first direction wavelength channels, namely CH3, CH6, CH9, and CH12. The centers of these four symmetrical first direction wavelength channels are located between CH7 and CH8 and are relatively large in interval, so the four-wave mixing effect caused is relatively weak.

Through the above scheme 3, the six first direction wavelength channels and the six second direction wavelength channels will not cause serious non-degenerate four-wave mixing (i.e., non-degenerate four-wave mixing) when transmitted in the optical fiber. Theoretical simulation found that based on the distribution of scheme 3, the performance degradation caused by four-wave mixing can be reduced by about 20 times compared with the distribution (configuration) of Figure 3. For further effect analysis, please refer to the introduction of scheme 2 above, which will not be repeated here.

Solution 4: The intervals between the six wavelength channels propagating in the same direction are different. Specifically, based on FIG. 6a or FIG. 6b , the first direction and the second direction of the three wavelength channel pairs are flipped.

Based on whether the wavelength of the first direction wavelength channel in the first wavelength channel pair is longer or shorter than that of the second direction wavelength channel, the following two solutions can be divided.

Solution 4.1: The wavelength of the wavelength channel in the first direction is shorter than the wavelength of the wavelength channel in the second direction.

Please refer to FIG. 9a, which is a schematic diagram of another wavelength channel distribution provided in the present application. The schematic diagram of the wavelength channel distribution corresponds to the serial number 28 in the above Table 2. The 6 first-direction wavelength channels and the 6 second-direction wavelength channels are distributed in the order of wavelength from small to large as follows: first-direction wavelength channel, second-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel, second-direction wavelength channel, first-direction wavelength channel. Or it can also be expressed as the distribution of the 6 first-direction wavelength channels: CH1-CH4-CH6-CH7-CH10-CH12, and the distribution of the 6 second-direction wavelength channels: CH2-CH3-CH5-CH8-CH9-CH11. For the introduction of the wavelength channel pairs, please refer to the description in the aforementioned scheme 1.1, which will not be repeated here. It can be understood that the above FIG. 9a is equivalent to swapping the first direction and the second direction of CH3 and CH4, the first direction and the second direction of CH5 and CH6, the first direction and the second direction of CH9 and CH10, and the first direction and the second direction of CH11 and CH12 in the above FIG. 6a. The swapping of the first direction and the second direction can be understood as the first direction wavelength channel in FIG. 6a and the second direction wavelength channel in FIG. 9a.

Taking CH8, CH9, CH10, CH11 and CH12 located in the ZDW region as an example, in the ZDW region, the second direction wavelength channels are distributed in the 8th, 9th and 12th, and these three second direction wavelength channels will not produce four-wave mixing; the first direction wavelength channels are distributed in the 10th and 11th, and these two first direction wavelength channels will not produce four-wave mixing. Moreover, the 6 first direction wavelength channels satisfy: there are no four symmetrical first direction wavelength channels whose centers are located in the ZDW region, nor are there: three first direction wavelength channels with the same intervals and at least two of which are located in the ZDW region. The 6 second direction wavelength channels satisfy: there are no four symmetrical second direction wavelength channels whose centers are located in the ZDW region, nor are there: three second direction wavelength channels with the same intervals and at least two of which are located in the ZDW region. It can also be understood that the distribution of the 6 first direction wavelength channels and the 6 first direction wavelength channels based on the above scheme 4.1 satisfies the above conditions 1 and 2. Therefore, the distribution based on the above scheme 4.1 helps to further weaken or suppress the four-wave mixing effect.

It should be noted that in the wavelength channel distribution of the above scheme 4.1, there are four symmetrical first-direction wavelength channels among the N first-direction wavelength channels, namely, CH4, CH5, CH10, and CH11, and the centers of these four symmetrical first-direction wavelength channels are located between CH7 and CH8, causing a weak four-wave mixing effect. Similarly, there are four symmetrical second-direction wavelength channels among the N second-direction wavelength channels, namely, CH2, CH3, CH8, and CH9, and the centers of these four symmetrical second-direction wavelength channels are located between CH5 and CH6, causing a weak four-wave mixing effect.

Solution 4.2: The wavelength of the wavelength channel in the first direction is longer than the wavelength of the wavelength channel in the second direction.

Please refer to FIG. 9b, which is a schematic diagram of another wavelength channel distribution provided by the present application. The schematic diagram of the wavelength channel distribution corresponds to the serial number 50 in the above Table 2. The 6 first direction wavelength channels and the 6 second direction wavelength channels are distributed in the order of wavelength from small to large as follows: second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel. Or it can also be expressed as the distribution of the 6 first direction wavelength channels: CH2-CH3-CH5-CH8-CH9-CH11, and the distribution of the 6 second direction wavelength channels: CH1-CH4-CH6-CH7-CH10-CH12. It can be understood that FIG9b is equivalent to flipping the first and second directions of CH3 and CH4, CH5 and CH6, CH9 and CH10, and CH11 and CH12 in FIG6b.

The distribution of 6 first direction wavelength channels and 6 second direction wavelength channels based on the above scheme 4.2 can also weaken or suppress the four-wave mixing effect of the optical fiber. For specific analysis, please refer to the introduction of scheme 4.1, which will not be repeated here.

It should be noted that in the wavelength channel distribution of the above scheme 4.2, there are four symmetrical second direction wavelength channels among the six second direction wavelength channels, namely CH4, CH5, CH10, and CH11. The centers of these four symmetrical second direction wavelength channels are located between CH7 and CH8, and the intervals are large. Therefore, the four-wave mixing effect caused is weak. Similarly, there are four symmetrical first direction wavelength channels among the N first direction wavelength channels, namely CH2, CH3, CH8, and CH9. The centers of these four symmetrical first direction wavelength channels are located between CH5 and CH6, and the intervals are large. Therefore, the four-wave mixing effect caused is weak.

Through the above scheme 4, the six first direction wavelength channels and the six second direction wavelength channels will not cause serious non-degenerate four-wave mixing (i.e., non-degenerate four-wave mixing) when transmitted in the optical fiber. Theoretical simulation found that based on the distribution of scheme 3, the performance degradation caused by four-wave mixing can be reduced by about 20 times compared with the distribution (configuration) of Figure 3. For further effect analysis, please refer to the introduction of scheme 2 above, which will not be repeated here.

It can be understood that the distribution of wavelength channels for weakening or suppressing four-wave mixing given above is only an example, and any distribution of wavelength channels that can meet the above conditions 1 and/or conditions 2 and/or conditions 3 and/or conditions 4 can weaken or suppress four-wave mixing. For example, the wavelength channel distributions of sequence numbers 7/15/18/39/46/50 in Table 2 above can also effectively weaken or suppress the four-wave mixing effect of the wavelength channels in the optical fiber.

2. First Node and Second Node

In a possible implementation, the first node includes N first optical transceivers and a first combiner/demultiplexer, and the first end of the first combiner/demultiplexer is connected to the N first optical transceivers; the second node includes N second optical transceivers and a second combiner/demultiplexer, and the first end of the second combiner/demultiplexer is connected to the N second optical transceivers. The second end of the first combiner/demultiplexer is connected to the optical fiber between the first node and the second node, and the second end of the second combiner/demultiplexer is connected to the optical fiber between the first node and the second node. In other words, the second end of the second combiner/demultiplexer and the second end of the second combiner/demultiplexer can be connected through a trunk optical fiber. In this way, only N first optical transceivers and N second optical transceivers are needed to transmit 2N wavelength channels, which helps to reduce the number of first optical transceivers and/or the number of second optical transceivers included in the optical communication system.

Further, optionally, the number of ports of the first end and the second end of the first combiner/demultiplexer is N:1, and the number of ports of the first end and the second end of the second combiner/demultiplexer is N:1. N first-direction wavelength channels are sent through the N ports of a first combiner/demultiplexer and N second-direction wavelength channels can be received, that is, the N ports of a first combiner/demultiplexer realize the transmission of 2N wavelength channels, which helps the first combiner/demultiplexer save 50% of the number of ports. Further, N second-direction wavelength channels are sent through the N ports of a second combiner/demultiplexer and N first-direction wavelength channels are received, that is, the N ports of a second combiner/demultiplexer realize the transmission of 2N wavelength channels, which helps the second combiner/demultiplexer save 50% of the number of ports.

Exemplarily, the number of ports at the first end of the first combiner/demultiplexer (or the second combiner/demultiplexer) is N, and the number of ports at the second end is 1; or, the number of ports at the first end of the first combiner/demultiplexer (or the second combiner/demultiplexer) is n×N, n is an integer greater than 1, and the number of ports at the second end is 2, and so on. It should be noted that, in order to prevent the first combiner/demultiplexer from being unable to be used normally due to a number failure of one or several ports at the first end, the number of ports at the first end of the first combiner/demultiplexer (or the second combiner/demultiplexer) may also be any integer greater than N and less than n×N. Similarly, the second end of the first combiner/demultiplexer (or the second combiner/demultiplexer) may also include more than 2 ports.

Please refer to Figure 10, which is a schematic diagram of the connection relationship between a first node and a second node provided in the present application. In this example, N=6 is taken as an example. The first node includes 6 first optical transceivers and a first MUX/DMUX. Furthermore, the first node may also include a DU/CU. The second node includes a second MUX/DMUX and 6 second optical transceivers. Further, optionally, the second node may also include 6 AAUs. The first end of the first MUX/DMUX is connected to the 6 first optical transceivers, the second end of the first MUX/DMUX is connected to the second end of the second MUX/DMUX through a trunk optical fiber, the first end of the second MUX/DMUX can be connected to the 6 second optical transceivers through 6 home optical fibers, and the 6 first optical transceivers are all connected to the DU/CU. The 6 second optical transceivers can be connected to the AAU.

It should be noted that the DU/CU, the first optical transceiver and the first MUX/DMUX may be integrated, or any two of them may be integrated, or they may be three independent physical entities. The AAU and the second optical transceiver may be integrated, or they may be two independent physical entities.

In a possible implementation, a first optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel, which are adjacent to the first direction wavelength channel and the second direction wavelength channel corresponding to the same first optical transceiver. Further, a second optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel, which are adjacent to the first direction wavelength channel and the second direction wavelength channel corresponding to the same second optical transceiver. It can also be understood that the adjacent first direction wavelength channel and the second direction wavelength channel are used for bidirectional transmission between the first node and the second node, and a first optical transceiver corresponds to a wavelength channel pair, and a second optical transceiver also corresponds to a wavelength channel pair. Or it can also be understood that the first optical transceiver is used to send the first direction wavelength channel and to receive the second direction wavelength channel belonging to the same wavelength channel pair. The second optical transceiver is used to receive the first direction wavelength channel and to send the second direction wavelength channel belonging to the same wavelength channel pair. Each of the first optical transceiver and the second optical transceiver is pre-configured with a corresponding first direction wavelength channel and a second direction wavelength channel. By realizing bidirectional transmission through a single fiber, the number of trunk optical fibers and the number of household optical fibers can be saved, thereby improving the resource utilization of optical fibers.

Taking the above scheme 1.1 as an example, in the first direction: DU/CU sends CH1, CH3, CH5, CH7, CH9 and CH11 through 6 first optical transceivers, and after multiplexing through the first MUX, they are transmitted on the trunk optical fiber. After reaching the second MUX/DMUX, they are demultiplexed by the second DMUX and transmitted to the corresponding second optical transceivers respectively, and then transmitted to the connected AAU through the second optical transceiver. In the second direction: AAU sends CH2, CH4, CH6, CH8, CH10 and CH12 through the second optical transceiver. After multiplexing through the second MUX, they are transmitted on the trunk optical fiber. After reaching the first MUX/DMUX, they are demultiplexed by the first DMUX and output to DU/CU. The transmission process of other schemes is not listed here one by one. Specifically, the distribution of 6 first-direction wavelength channels and 6 second-direction wavelength channels can be replaced with the corresponding distribution scheme.

In a possible implementation, the first optical transceiver may be, for example, a first (Bi-directional, BiDi) optical module (or BiDi transceiver module, or BiDi bidirectional transmission module), or may be a first optical fiber circulator. The second optical transceiver may be, for example, a second BiDi optical module, or may be a second optical fiber circulator. Further, optionally, the first BiDi optical module and the second BiDi optical module may be the same, and for the convenience of subsequent description, may be collectively referred to as BiDi optical modules. The first optical fiber circulator and the second optical fiber circulator may also be the same, and for the convenience of subsequent description, may be collectively referred to as optical fiber circulators.

Among them, the BiDi optical module is a bidirectional optical module. The BiDi optical module has only one port. Through the filtering in the BIDI module, it can simultaneously complete the transmission of one wavelength channel and the reception of another wavelength channel. For example, the first node side transmits a 1310nm wavelength channel and receives a 1330nm wavelength channel at the same time. The wavelength channel used by the second node is exactly the opposite of the first node side, that is, it transmits a 1330nm wavelength channel and receives a 1310nm wavelength channel.

As shown in Figure 11, it is a schematic diagram of the structure of a fiber circulator provided by the present application. The fiber circulator can realize bidirectional transmission of wavelength channels on the fiber side. The fiber circulator is a multi-port non-reciprocal optical device, and the wavelength channel can only propagate in one direction. Exemplarily, both reception and transmission are processed through two ports. If wavelength channel 1 is input from port 1, it is output from port 2; if wavelength channel 2 is input from port 2, it will be output from port 3. If wavelength channel 2 is input from port 2, the loss is large when it is output from port 1. Similarly, when wavelength channel 1 is input from port 3, the loss is large when it is output from port 1 or port 2. The structure of the fiber circulator is relatively simple, and only one fiber optic connector is needed to connect to the DU/CU or AAU, which helps to reduce the volume of the optical communication system.

Please refer to Figure 12, which is a schematic diagram of the connection relationship between a first optical fiber circulator and a second optical fiber circulator provided by the present application. The first optical transceiver may include a first transmitting port and a first receiving port, the second optical transceiver may include a second transmitting port and a second receiving port, the three ports of the first optical circulator are respectively connected to the first transmitting port, the first receiving port and the second optical fiber circulator, and the three ports of the second optical fiber circulator are respectively connected to the second transmitting port, the second receiving port and the first optical fiber circulator.

In the various embodiments of the present application, unless otherwise specified or provided for by logic, the terms and/or descriptions of different embodiments are consistent and may be referenced to each other, and the technical features of different embodiments may be combined to form new embodiments according to their inherent logical relationships.

In this application, "at least one" means one or more, and "more than one" means two or more. "And/or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and/or B can mean: A exists alone, A and B exist at the same time, and B exists alone, where A and B can be singular or plural. In the text description of this application, the character "/" generally indicates that the previous and next associated objects are in an "or" relationship. In the formula of this application, the character "/" indicates that the previous and next associated objects are in a "division" relationship. In the present application, the symbol "(a, b)" represents an open interval, the range is greater than a and less than b; "[a, b]" represents a closed interval, the range is greater than or equal to a and less than or equal to b; "(a, b]" represents a semi-open and semi-closed interval, the range is greater than a and less than or equal to b; "(a, b]" represents a semi-open and semi-closed interval, the range is greater than a and less than or equal to b. In addition, in the present application, the word "exemplarily" is used to indicate an example, illustration or description. The embodiments or designs described as "exemplary" in the present application should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Alternatively, it can be understood that the use of the word "exemplary" is intended to present the concept in a specific way and does not constitute a limitation on the present application.

It is to be understood that the various digital numbers involved in the present application are only for the convenience of description and are not intended to limit the scope of the embodiments of the present application. The size of the sequence number of the above-mentioned processes does not mean the order of execution, and the order of execution of each process should be determined by its function and inherent logic. The terms "first", "second" and the like are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. In addition, the terms "including" and "having" and their variations are intended to cover non-exclusive inclusions, for example, including a series of steps or units. Methods, systems, products or equipment are not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or equipment.

Claims (23)

1. An optical communication system, characterized in that it includes a first node, a second node, and an optical fiber for connecting the first node and the second node, and the zero dispersion wavelength ZDW of the optical fiber belongs to one ZDW area; The optical fiber is used to transmit N first direction wavelength channels and N second direction wavelength channels, where N is an integer greater than 2, and at least two first direction wavelength channels and at least one second direction wavelength channel are located where The ZDW area, or at least two second direction wavelength channels and at least one first direction wavelength channel are located in the ZDW area, and the first direction wavelength channels and the second direction wavelength channels in the ZDW area are mixed and distributed, and the The first direction wavelength channel is the wavelength channel sent by the first node to the second node, and the second direction wavelength channel is the wavelength channel sent by the second node to the first node. 2. The system of claim 1, wherein the distribution of the N first direction wavelength channels and the N second direction wavelength channels satisfies any one or more of the following: Among the N first direction wavelength channels, there is no: the centers of four symmetrical first direction wavelength channels are located in the ZDW region; Among the N second direction wavelength channels, there is no center of four symmetrical second direction wavelength channels located in the ZDW region. 3. The system according to claim 1 or 2, wherein the distribution of the N first direction wavelength channels and the N second direction wavelength channels satisfies any one or more of the following: Among the N first direction wavelength channels, there are no: three first direction wavelength channels with the same spacing and at least two of which are located in the ZDW region; Among the N second direction wavelength channels, there are not three second direction wavelength channels with the same spacing and at least two of which are located in the ZDW region. 4. The system according to any one of claims 1 to 3, characterized in that the minimum frequency spacing of the mixed distribution of the N first direction wavelength channels and the N second direction wavelength channels is equal to 800 GHz. . 5. The system according to any one of claims 1 to 3, characterized in that the minimum frequency spacing of the mixed distribution of the N first direction wavelength channels and the N second direction wavelength channels is equal to 400 GHz. . 6. The system according to any one of claims 1 to 5, wherein the wavelength intervals of the N first direction wavelength channels are the same, and the wavelength intervals of the N second direction wavelength channels are the same. 7. The system of claim 6, wherein N=6; The N first direction wavelength channels and the N second direction wavelength channels are distributed in order of wavelength from small to large as: First direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first Directional wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel; or, Second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel Directional wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel. 8. The system according to any one of claims 1 to 5, characterized in that the wavelength intervals of the N first direction wavelength channels are different; and/or the wavelength intervals of the N second direction wavelength channels are different. different. 9. The system of claim 8, wherein N=6; The N first direction wavelength channels and the N second direction wavelength channels are distributed in order of wavelength from small to large as: First direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first Directional wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel; or, Second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel Directional wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel. 10. The system of claim 8, wherein N=6; The N first direction wavelength channels and the N second direction wavelength channels are distributed in order of wavelength from small to large as: First direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second direction wavelength channel Directional wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel; or, The second direction wavelength channel, the first direction wavelength channel, the first direction wavelength channel, the second direction wavelength channel, the second direction wavelength channel, the first direction wavelength channel, the second direction wavelength channel, the first direction wavelength channel, the first Directional wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel. 11. The system of claim 8, wherein N=6; The N first direction wavelength channels and the N second direction wavelength channels are distributed in order of wavelength from small to large as: First direction wavelength channel, second direction wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel, first direction wavelength channel, second direction wavelength channel, second Directional wavelength channel, first direction wavelength channel, second direction wavelength channel, first direction wavelength channel; or, The second direction wavelength channel, the first direction wavelength channel, the first direction wavelength channel, the second direction wavelength channel, the first direction wavelength channel, the second direction wavelength channel, the second direction wavelength channel, the first direction wavelength channel, the first Directional wavelength channel, second direction wavelength channel, first direction wavelength channel, second direction wavelength channel. 12. The system according to any one of claims 7, 9 to 11, wherein the five longest wavelengths among the mixed distribution of 6 first direction wavelength channels and 6 second direction wavelength channels are located in the ZDW area. 13. The system according to any one of claims 1 to 12, wherein the optical fiber includes a standard single-mode optical fiber, and the ZDW region of the standard single-mode optical fiber is in the range of [1300 nanometers, 1324 nanometers ]. 14. The system according to any one of claims 1 to 13, wherein the first node includes N first optical transceivers and a first combiner/demultiplexer, and the second node includes N a second optical transceiver and a second combiner/demultiplexer; The first end of the first combiner/demultiplexer is connected to the N first optical transceivers, and the second end of the first combiner/demultiplexer is connected to the first node and the second node. fiber optic connections between; The first end of the second combiner/demultiplexer is connected to the N second optical transceivers, and the second end of the second combiner/demultiplexer is connected to the first node and the second node. fiber optic connection between them. 15. The system of claim 14, wherein a first optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel, and the first direction wavelength corresponding to the same first optical transceiver The channel is adjacent to the second direction wavelength channel; and/or, A second optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel, and is adjacent to the first direction wavelength channel and the second direction wavelength channel corresponding to the same second optical transceiver. 16. The system of claim 14 or 15, wherein the first optical transceiver includes a first fiber optic circulator and the second optical transceiver includes a second fiber optic circulator. 17. The system according to any one of claims 14 to 16, wherein the number of ports at the first end and the second end of the first combiner/demultiplexer is N:1, and the number of ports at the first combiner/demultiplexer is N:1, and the second combiner/demultiplexer has The number of ports at the first end and the second end of the demultiplexer is N:1, and N is a positive integer. 18. The system according to any one of claims 1 to 17, wherein the first node includes a distribution unit DU and/or a central unit CU, and the second node includes an active antenna unit AAU. 19. A first node, characterized in that it includes: N first optical transceivers, the N first optical transceivers are used to send N first direction wavelength channels, and are used to receive N second direction wavelength channels, and the N is an integer greater than 2; Wherein, the N first direction wavelength channels and the N second direction wavelength channels are transmitted through optical fibers, the zero dispersion wavelength ZDW of the optical fiber belongs to one ZDW area, at least two first direction wavelength channels or at least two The second direction wavelength channel is located in the ZDW area. The first direction wavelength channel and the second direction wavelength channel in the ZDW area are mixed and distributed. The first direction wavelength channel is for the first node to transmit to the second node. wavelength channel, and the second direction wavelength channel is the wavelength channel sent by the second node to the first node. 20. The node according to claim 19, wherein the first node further includes a first combiner/demultiplexer, and a first end of the first combiner/demultiplexer is connected to the N first The optical transceiver is connected, and the second end of the first combiner/demultiplexer is used to connect with the optical fiber. 21. The node according to claim 19 or 20, characterized in that a first optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel, and the first optical transceiver corresponding to the same first optical transceiver corresponds to a first direction wavelength channel and a second direction wavelength channel. The directional wavelength channel and the second directional wavelength channel are adjacent. 22. The node according to any one of claims 19 to 21, wherein the first optical transceiver includes a first fiber optic circulator. 23. The node according to any one of claims 20 to 22, wherein the number of ports at the first end and the second end of the first combiner/demultiplexer is N:1.